A low rate of end-stage kidney disease in membranous nephropathy: A single centre study over 2 decades

Introduction Membranous nephropathy is the commonest cause of nephrotic syndrome in non-diabetic Caucasian adults over the age of 40 years. Primary membranous nephropathy is limited to the kidneys. Clinical management aims to induce remission, either spontaneously with supportive care, or with immunosuppression. Here, we describe the natural history of this condition in a large tertiary centre in the UK. Methods 178 patients with primary membranous nephropathy were identified over 2 decades. We collected data on demographics, baseline laboratory values, treatment received and outcomes including progression to renal replacement therapy and death. Analysis was performed on the whole cohort and specific subgroups. Univariate and multivariate Cox regression was also performed. Results Median age was 58.3 years with 63.5% male. Median baseline creatinine was 90μmol/L and urine protein-creatinine ratio 664g/mol. Remission (partial or complete) was achieved in 134 (75.3%), either spontaneous in 60 (33.7%) or after treatment with immunosuppression in 74 (41.6%), and of these 57 (42.5%) relapsed. Progression to renal replacement therapy was seen in 10.1% (much lower than classically reported) with mortality in 29.8%. Amongst the whole cohort, those who went into remission had improved outcomes compared to those who did not go into remission (less progression to renal replacement therapy [4.5% vs 28%] and death [20.1% vs 67%]. Those classified as high-risk (based on parameters including eGFR, proteinuria, serum albumin, PLA2R antibody level, rate of renal function decline) also had worse outcomes than those at low-risk (mortality seen in 52.6% vs 10.8%, p<0.001). The median follow-up period was 59.5 months. Conclusion We provide a comprehensive epidemiologic analysis of primary membranous nephropathy at a large tertiary UK centre. Only 10.1% progressed to renal replacement therapy. For novelty, the KDIGO risk classification was linked to outcomes, highlighting the utility of this classification system for identifying patients most likely to progress.


Introduction
There have been very few studies assessing large groups of patients with MN over selected time periods. We have undertaken a 20-year retrospective observational study of PMN patients diagnosed at our tertiary renal centre. Our aim was to describe the epidemiology, baseline characteristics, natural disease course, variation in management, and outcomes for this cohort. We believe this provides valuable real-world insight into PMN in a large patient dataset. In addition, we have for the first-time categorised patients as having either low, medium, or highrisk of progression based on KDIGO criteria published in the 2021 glomerulonephritis guidelines and described real-world outcomes.

Patient population
This was a retrospective cohort study conducted on patients diagnosed with PMN at our tertiary renal centre (Salford Royal Hospital, UK), encompassing a catchment population of 1.55 million, between January 2000 and December 2019. The population is largely urban and includes a mixture of affluent areas and those with increased social deprivation.
The Salford Royal kidney biopsy database was screened for patients who were diagnosed with MN between January 2000 and December 2019. Of 238 initially identified, exclusions occurred for the following reasons: diagnosis of secondary MN (39), follow up not at our centre (17), alternative diagnosis (3) or biopsy from a transplant (1). The final study population was 178 patients (Fig 1).
The date of kidney biopsy was used as the study baseline, and all patients were followed until they reached a study endpoint which included (i) commencement of RRT, (ii) death, (iii) end of analysis period (31 December 2020) or (iv) lost to follow up or last documented clinic appointment.
Data on baseline characteristics; laboratory results including creatinine, estimated glomerular filtration rate (eGFR), haemoglobin, albumin, calcium, phosphate, urine protein-creatinine ratio (uPCR); treatment received (including renin-angiotensin system (RAS) blockade and initial immunosuppression); date of initiation of RRT (either transplantation or dialysis); and mortality were gathered from the electronic patient record (EPR). All baseline characteristics and laboratory results were those obtained at the time of or +/-6 months of biopsy.
A comorbidity of hypertension was defined as a history of hypertension recorded in hospital records, a recorded high blood pressure and/or receiving antihypertensive therapy. A comorbidity of cardiovascular disease included a history of ischaemic heart disease, heart failure, cerebrovascular disease, or peripheral vascular disease. eGFR values were calculated using the CKD Epidemiology Collaboration (CKD-EPI) formula.
Partial remission was defined as a return to baseline eGFR and uPCR <350g/mol with at least 50% reduction from peak value. Complete remission was defined as return to baseline eGFR and uPCR<25g/mol [13].

Ethical considerations
The study complies with the declaration of Helsinki and as indicated by the NHS Health Research Authority online tool http://www.hra-decisiontools.org.uk/research this study was not considered research requiring research ethics committee review as it was a retrospective observational study using measurements routinely collected and using fully anonymised and secondary use of data. The need for individual patient consent was waived by the Research and Innovation committee of the Northern Care Alliance NHS Group. The committee granted study approval and registered the study (Ref: ID S21HIP41) after approving the methodological protocol as outlined above.

Statistical analysis
Analysis of baseline characteristics, comorbidities, remission and relapse rates, requirement for RRT, mortality, use of RAS blockade and use and effect of immunosuppression was undertaken in the total cohort. Continuous non-parametric variables are presented as median (interquartile range) and the Mann-Whitney U-test was used to assess between group differences. Categorical data are expressed as number (percentage), and the Chi-square test was used to assess between group differences.
The association of baseline variables with requirement for RRT and mortality was calculated using univariate and multivariate Cox proportional hazard models to determine hazard ratios (HR), 95% confidence intervals (CIs) and statistical significance.
The effect of immunosuppression was determined by comparing those who received immunosuppression and those who did not. The most frequently used specific type of immunosuppression was Modified Ponticelli, and this group was compared against all other types of immunosuppression combined. Participants were also stratified according to their risk of progression (low, medium, high) based on recently published KDIGO guidelines (2021) [7], and also based on the degree of proteinuria at presentation.
Statistical analysis was performed using IBM SPSS (version 24, University of Manchester).

Univariate and multivariate Cox regression assessing factors associated with all-cause mortality and progression to RRT
To determine baseline characteristics or laboratory values which were associated with all-cause mortality or need for RRT we performed Cox regression analysis to generate hazard ratios with 95% confidence intervals and p-values (  It is worth noting that use of immunosuppression being related to increased risk of progression to RRT is likely due to confounding factors such as more severe form of disease at presentation influencing the decision to use immunosuppression.

Modified Ponticelli vs other immunosuppression
Of those who received immunosuppression, the largest category was MP (n = 36). The baseline characteristics, laboratory values and outcomes for those who received MP and those who received other forms of immunosuppression can be seen in Table 3. When comparing these 2 groups, there were fewer relapses (38.7% vs 66.7%, p = 0.02) in those who received MP, increased remission rates (88.9% vs 71.9%, p = 0.053), reduced progression to RRT (2.8% vs 22.8%, p = 0.02), lower mortality (11.1% vs 36.2%, p = 0.007), less co-existing hypertension (33.3% vs 58.6%, p = 0.017), and higher calcium levels ( There were 58 patients who received non-MP immunosuppression. Fig 2 depicts the full range of different immunosuppression types administered which includes prednisolone in combination with CNIs, MMF, cyclophosphamide and azathioprine; along with CNIs alone, cyclophosphamide alone and rituximab alone. The variety of immunosuppression use reflects the long study period in which immunosuppression use was less standardised prior to the introduction of KDIGO guidelines in 2012.

Remission rates (partial, complete or no remission)
Within the total cohort, 39 (22.5%) patients did not achieve remission, 65 (37.6%) achieved partial remission and 69 (39.9%) achieved complete remission (Table 4). Spontaneous remission was seen in 60 (33.7%), whilst remission after treatment with immunosuppression was seen in 74 (41.6%). Remission data was not available for 5 patients due to lack of available data. Baseline characteristics were compared between those who did not achieve remission and those who achieved complete remission. There were several significant differences between these groups (Table 4) including mortality (67% vs 7%, p<0.001) and progression to RRT (28% vs 1%, p<0.001). This is also shown in the Kaplan-Meier curves in Fig 3 for

Risk of progression (low/ medium/ high)
Patients were categorised as having either low, medium or high risk of progression based on KDIGO criteria published in the 2021 glomerulonephritis guidelines [7]. The criteria include eGFR, albumin, proteinuria and other factors indicating high-risk of progression such as anti-PLA2R value. We identified 83 patients at low-risk of progression, 15 patients with mediumrisk of progression and 76 patients at high-risk of progression ( Table 5).

Timing of biopsy
Finally, we also split the cohort according to timing of biopsy (2000-2011 and 2012-2019).
The results can be seen in S2 Table. The high-risk group was compared with the low-risk group. The high-risk group was older (median age at diagnosis 61. 2  .001), more likely to progress to RRT (8.1% vs 3.7%, p = 0.012) and more likely to die (52.6% vs 10.8%, p<0.001). Treatment approaches were similar across the 2 groups with 39 (51.3%) receiving immunosuppression in the high-risk group, and 46 (55.4%) receiving immunosuppression in the low-risk group. Of those who received immunosuppression in the highrisk group, 12 (30.1%) received MP whereas 20 (43.5%) received MP in the low-risk group. The high-risk group also had a shorter duration of follow up (42 months [19.5-118] vs 74 months [43-99], p = 0.009) likely reflecting the increased rate of outcomes observed in this group. Kaplan-Meier curves for overall survival and freedom from RRT for those with low, medium, and high-risk of progression can be seen in Fig 4(A) and 4(B).

Discussion
This is a large retrospective epidemiological study for PMN which provides important realworld information about the natural history of this condition. This is also, to the best of our knowledge, the first study to categorise patients into low, medium, and high-risk of progression as per the 2021 KDIGO guidelines. Key findings from the study are that i) progression to RRT was only seen in a minority (10.1%), in contrast to the previous literature, ii) most patients presented with low serum albumin and nephrotic range proteinuria (median serum albumin 26g/L and median uPCR 664g/mol); and iii) the majority went into remission (spontaneous 33.7%, and after treatment 41.6%)-but of these a significant number relapsed (42.5%). Patients who received the modified Ponticelli immunosuppressive regime had significantly better outcomes than those who received other forms of immunosuppression. The value in risk stratifying patients according to their risk of progression was confirmed; those at high risk had far worse outcomes compared to those at low risk.
There are few long-term prospective or retrospective studies of PMN reported in the literature. Rozenberg et al. [14] performed a retrospective study of PMN in 2018 which analysed 16 patients considered as being at high-risk of progression and treated with immunosuppression, and 21 patients considered as low risk who received supportive care. Remission rates were 68.7% in the high-risk group (in comparison to 56% in our cohort), and 90.4% in the low-risk group (in comparison to 97.6% in our cohort). Progression to RRT was seen in 25% of the high-risk group (compared to only 8.1% in our cohort). A group from New Zealand assessed the outcomes of MN (amongst other glomerulonephritidies) in a cohort who were initially  [16].
A retrospective study in London identified 148 MN patients between 1995 and 2015 [17]. It found that those who presented with a higher creatinine were more likely to develop RRT, and surprisingly, those who progressed to RRT were more likely to have a higher serum albumin at presentation than those who did not. Black patients (comprising 24% of their cohort) had a worse outcome and poorer response to treatment. Within this cohort 36% were White, whereas 88.8% were of Caucasian ethnicity in our cohort. This suggests that there may be outcome differences based on ethnicity that requires further evaluation in multi-ethnic cohorts.
Other retrospective studies have been undertaken in China [18], Korea [19], Japan [20,21], Australia and New Zealand [22], USA [23] and the UK [24]. In an elderly cohort from Korea [19], immunosuppression use was found to increase the likelihood of adverse renal outcomes and increased rates of infection, whereas RAS blockade reduced both, suggesting that use of immunosuppression should be carefully considered in the elderly. A UK study including patients diagnosed between 1980 and 2010 identified 128 patients with PMN [24]. This study reported similar remission rates to our study, however of the 21.9% that did not achieve remission, 75% of these patients went on to develop ESKD in comparison to only 28% in our study. This is likely due to the earlier time period of the study in an era when RAS blockade was not universal and immunosuppression use was more limited. The MP regimen was established into clinical practice in the late 1990s after evidence emerged for the beneficial effect of alternating monthly cycles of steroids and either chlorambucil or cyclophosphamide [25]. Our study suggests that changes in treatment, in particular with standardisation of practice with immunosuppression use, has improved outcomes for patients with PMN in a real-world setting.
We observed a wide variety of types of immunosuppression used, with MP (38 patients) being the largest category. The reason for the disparity in immunosuppression types can be explained by the long time-period of the study (20 years) with KDIGO guidelines published in 2012 likely contributing to a greater degree of standardisation in clinical practice thereafter. It is interesting to note the superior outcomes achieved with MP compared to other immunosuppression when examined in the immunosuppressed sub-group. It is also important to bear in mind that not all patients who start MP treatment complete the 6-month course. These results should be interpreted with caution as the comparison contains many different types of immunosuppression within the non-MP cohort. A better comparison would be MP vs another specific type of immunosuppression treatment such as calcineurin inhibitors or rituximab, but we were unable to perform this analysis due to the low numbers attributed to these individual immunosuppression treatment types within our cohort. Furthermore, the MP results may be confounded by intention to treat bias-it is possible that we did not give MP to those who were felt to be more susceptible to its risks, for example the frail elderly, who more likely to have worse outcomes regardless of the immunosuppression type used.
Our centre first began testing anti-PLA2R antibody levels in 2012 and as such our anti-PLA2R data is far from complete. A future study would aim to collect more comprehensive anti-PLA2R data and look at factors such as whether antibody titres correlate with likelihood of remission, severity of disease and whether levels can predict risk of relapse, as reported in previous studies [26,27].
One area that our study did not assess was thrombo-embolic risk with IMN. Zou et al. assessed the incidence of venous thromboembolism (VTE) and arterial thromboembolism in a cohort of 766 patients in China [28]. The highest risk was seen within the first 6 months after diagnosis, reaching 8.0% for arterial thromboembolism and 7.2% for venous thromboembolism at 5 years post diagnosis. Hypoalbuminaemia was found to be the dominant independent risk factor for VTE, with nephrotic syndrome present in 81.5% of patients who had a VTE.
Finally, the study time period was largely before the sodium-glucose co-transport-2 inhibitors (SGLT2i) era. Given the growing evidence for the use of SGLT2i in proteinuric kidney disease regardless of cause [29], we suggest that they should be considered for overall nephroprotection in patients with MN.

Conclusion
This large real-world study has identified several key points. Only 10.1% of our cohort progressed to ESKD during the follow up period, suggesting that outcomes for PMN are improving. Those who received MP had better outcomes (less relapse, less progression to RRT and reduced mortality) compared to those treated with other immunosuppression. We were not able to assess the use of rituximab due to the time-period of the study resulting in only 2 patients receiving it. A high rate of remission was observed (75.3%), although a significant number of these (42.9%) subsequently relapsed. Failure to achieve remission was associated with worse outcomes. Finally, when comparing patients by risk status according to the KDIGO guidelines those classified as at high-risk were more likely to progress to RRT and had a higher mortality, highlighting the clinical utility of this classification.